Fuel cells, which generate electric current by the electrochemical combination of hydrogen and oxygen, are well known. In one form of such a fuel cell, an anodic layer and a cathodic layer are separated by an electrolyte formed of a ceramic solid oxide. Such a fuel cell is known in the art as a Solid-Oxide Fuel Cell (SOFC). SOFC systems derive electrical power through a high-efficiency conversion process from a variety of fuels including natural gas, liquefied petroleum gas, ethanol, and other hydrocarbon and non-hydrocarbon fuels. Hydrogen, either pure or reformed from hydrocarbons, is flowed along the outer surface of the anode and diffuses into the anode. Oxygen, typically from air, is flowed along the outer surface of the cathode and diffuses into the cathode.
Each O2 molecule is split and reduced to two O−2 anions catalytically at the cathode/electrolyte interface. The oxygen anions diffuse through the electrolyte and combine with four hydrogen ions at the anode/electrolyte interface to form two molecules of water. The anode and the cathode are connected externally through a load to complete the circuit whereby electrons are transferred from the anode to the cathode.
When hydrogen as a feed stock for the fuel cell is derived by “reforming” hydrocarbons, such as from gasoline, diesel fuel, natural gas, or methane, in the presence of limited oxygen, the reformate gas produced includes CO which is converted to CO2 at the anode/electrolyte interface. Since a single fuel cell is capable of generating a relatively small amount of voltage and wattage, in practice, it is known to stack a plurality of fuel cells together in electrical series.
Present anode supported SOFC technology uses a dense ceramic solid electrolyte membrane, for example yttria stabilized zirconia (YSZ), over which a cathode electrode consisting of an ionic conducting layer and a porous catalyst, such as a mixed ionic and electronic conductor (MIEC), is deposited. The cathode material is predominantly an electronic conductor with some ionic conductivity. At the cathode, oxygen is reduced and the ionic species pass through the electrolyte membrane to the anode where a fuel is oxidized to produce power. The resistance of the cathode, ohmic and polarization, plays a major role in the overall cell resistance and, therefore, can greatly affect electrochemical performance of the cell.
One prior art approach to decrease the cathode resistance (polarization) is to add a doped (Sm, Gd, Nd, Y, etc.) ceria based ionic conducting phase to the mixed ionic and electronic conductor (MIEC) material. While such cathodes may have an initially lower polarization resistance, the polarization resistance increases at elevated cell temperatures as low as about 800° C. In addition, such cathodes are structurally weak and tend to delaminate under certain conditions.
Another prior art approach is to modify the geometry of the cathode to a three layer configuration that includes an ionic conductor layer, a dual phase layer including the mixed ionic and electronic conductor (MIEC) material and ionic conducting material, and a mixed ionic and electronic conductor (MIEC) layer. Such a fuel cell is still susceptible to delamination and the power performance is not improved.
Therefore, cathodes of current solid oxide fuel cells have a high resistance (ohmic and polarization) and, thus, a relatively low power output due to poor adhesion, low ionic conductivity, and an insufficient microstructure of the cathode. Poor adhesion may result in the delamination of the cathode from the electrolyte surface, which may lead to a drastic reduction in output power and even cell failure.
What is needed in the art is a cathode of a solid oxide fuel cell with improved bonding to the electrolyte, that is highly electrocatalytic, and that is porous with contiguous electronic, ionic, and gas diffusion paths.
It is a principal object of the present invention to provide a cathode for a solid-oxide fuel cell that enables significant improvement of the power density of such fuel cell and that has an improved durability.
It is a further object of the invention to provide a method for low-temperature bonding of refractory ceramic layers.